Capacitive pressure transducers are frequently used to determine the pressure of gases and liquids for purposes of measurement or control. Some examples of prior art pressure transducers include a dual diaphragm differential pressure transducer as shown in U.S. Pat. No. 4,680,971 (the '971 patent) issued Oct. 14, 1986 and which is assigned to the assignee of the present application; a liquid capacitance pressure transducer as shown in U.S. Pat. No. 4,425,799, (the '799 patent) issued Jan. 17, 1984, and also assigned to the assignee of the present application; and a capacitive pressure transducer as shown in U.S. Pat. No. 4,388,668 (the '688 patent) issued Jun. 14, 1983, also assigned to the assignee of the present invention.
The '668 patent and the '799 patent each disclose a pressure transducer including two insulating plates, one of which is a diaphragm, bonded together and spaced apart by glass frit. The inside surface of each plate is coated with a conductive material. This configuration creates a variable capacitor which varies in capacitance according to pressure applied to the diaphragm.
The '971 patent discloses a capacitive pressure transducer having two insulating plates, both of which are diaphragms, bonded to either side of a central base plate also made of inert insulating material, and having one or more openings therethrough. The two diaphragms are coupled together, by means fluid between the diaphragms or by means of a rod extending through the central opening in the base plate and secured to the centers of the two diaphragms. The inside of at least one of the diaphragms and the facing surface(s) of the base plate are coated with a conductive material. With this configuration, the capacitance between the conductive plates varies when the two diaphragms flex together.
It is well known in the field, as is evidenced by the teaching of the '971 and '799 patents, that a fitting (or housing) around the variable capacitor can be used to direct the flow of pressure to the appropriate plate(s) of the capacitor. It is also well known that some means of sealing the capacitor to the fitting is needed.
The sealing means employed in prior art, as evidenced by the teaching of the '971 and '799 patents, is accomplished by using rubber rings or rubber gaskets to seal the fitting around the periphery of the outer surface(s) of the diaphragm(s). This practice works well with many gasses and liquids (hereinafter referred to jointly as fluids). There are, however, fluids including certain gases employed in refrigeration systems, which tend to accelerate the decomposition of rubber and like materials. In the event that pressure transducers using seals of rubber or like materials are used with these rubber deteriorating fluids they will in time deteriorate and leak.
Accordingly, a principal object of the present invention is to provide an alternative transducer to be used with fluids that accelerate the decomposition of rubber and like materials.